Until now, molybdenum mixed oxides have been obtained in the state of the art by precipitation methods, sol-gel methods or solid-state reactions.
Molybdenum mixed oxides are used in the state of the art as catalyst for chemical conversions. Conversions of alkyl compounds or alkene compounds to acrolein or derivatives thereof as well as a conversion of acrolein to acrylic acid may be named here as examples. The molybdenum mixed oxide catalysts according to the state of the art often do not display a sufficient activity in these reactions.
WO 2008/028681 and WO 2008/006565 A1 disclose a method for the production of nanocrystalline metal oxides or mixed metal oxides. There is no indication in these documents that special nanocrystalline molybdenum mixed oxides which are particularly well-suited as catalyst in particular for the conversion of acrolein to acrylic acid can be produced with the method.
A crystalline molybdenum mixed oxide can be obtained only with difficulty via conventional methods. Thus, G. A. Zenkovets et al., “The structural genesis of a complex (MoVW)5O14 oxide during thermal treatments and its redox behaviour at elevated temperatures”, Materials Chemistry and Physics, 103 (2007), 295-304, disclose that an Mo0.68V0.23W0.09Ox mixed oxide obtained via spray drying has an amorphous structure. This mixed oxide is present in the form of large aggregates approximately 5 μm in size. A partially nanocrystalline structure forms inside the aggregates due to subsequent calcining. A pure crystalline phase with crystallites more than 1000 nm in size forms only after prolonged thermal treatment at approximately 440° C. The production of a nanocrystalline molybdenum mixed oxide can thus be accomplished only with difficulty.
It is clear from O. Ovsiter et al., “Molybdenum oxide based partial oxidation catalyst Part 3”, Journal of Molecular Catalysis A: Chemical 185 (2002), 291-303, that a molybdenum mixed oxide does not display well-crystallized particles after thermal treatment. A crystalline phase, however, was able to be observed during a conversion of acrolein to acrylic acid, i.e. a crystallization first takes place during the oxidation reaction. Such a catalyst thus has a sufficient activity only after a prolonged reaction period.
A disadvantage of the molybdenum mixed oxides described in the state of the art is thus that a uniform particle size of the molybdenum mixed oxides cannot be obtained and a control of the crystallization, in particular with regard to the crystallite size, is not possible. The BET surface area of the molybdenum mixed oxides described in the state of the art is likewise mostly too small. A small particle size with as large as possible a BET surface area is desired, in particular for catalytic uses.